In complementary metal oxide silicon (CMOS) technology, a need to enhance the speed and increase the density of CMOS integrated circuits (IC's) has resulted in the evolution of transistor scaling, accompanied by progressively thinner gate dielectric oxide. Reduction in the thickness of a gate dielectric provides increased drive current, with resultant increased speed. In addition, a thinner gate dielectric offers enhanced control of channel charge, thereby reducing short channel effects. The fabrication of thinner gate oxides, however, presents gate leakage current and reliability issues. In particular, physically thinner gate oxides exhibit gate leakage current increasing exponentially with reduction in thickness.
The leakage current can be mitigated by introducing nitrogen atoms into the gate dielectric. One method of nitrogen atom introduction is to perform non-thermal nitridation (e.g., plasma nitridation) on the gate dielectric. Nitridation, however, introduces damage (e.g., plasma damage) to the top surface of the gate dielectric that can extend into the bulk of the film and result in nitrogen pile up at lower interface for thinner films. The damage can cause high gate leakage, threshold voltage shifts, or premature oxide breakdown when the devices are operating, as well as mobility and performance reduction. A post-nitridation high temperature (e.g., at or above 900° C.) re-oxidation (HT ReOx) can be performed on the gate dielectric to mitigate the plasma damage and improve GOI.
Exposure to air and airborne molecular contaminants, such as moisture and organics, following nitridation and/or re-oxidation of the gate dielectric can result in inadvertent oxide growth of the gate dielectric, which can increase the equivalent oxide thickness (EOT) of the gate dielectric. By way of example, a nitrided gate dielectric with an equivalent oxide thickness (EOT) of about 12-13 Å and containing about 6-8% nitrogen can be formed from a starting oxide film with a thickness of about 7-8 Å. Reducing the thickness of the starting oxide film below 7-8 Å to reduce the EOT of a nitrided gate dielectric is not practical. An oxide film with a thickness of about 7-8 Å includes about two monolayers of gate oxide atoms. A further reduction in the thickness of an oxide film would result in a monolayer (i.e., about 4 Å) oxide film. Single monolayer oxide films have increased roughness compared to dual monolayer oxide films. Roughness in the starting oxide film can degrade the performance of the nitrided gate dielectric. Another detrimental effect of inadvertent exposure to air and airborne molecular contaminants is increased and variable oxide growth across wafers and from wafer to wafer. This results in higher EOT (e.g., from AMC) for the first few wafers processed, especially as EOT is reduced below about 20 Å.